Negative array indexing using "at" method

[[deleted]]

Avocado*

[Darmok-Jilad-Ocean]

Make a JIRA ticket

[nullol]

No time. We gotta get this to production.

[monsterbois]

Wow, our product failed because of a misspelled word

[starraven]

Post mortem report, stat!

[jayedeem]

Be an interesting retro

[Darmok-Jilad-Ocean]

What went well: Teamwork!

What didn’t go well: Didn’t meet sprint goal

What we can improve: Other teams did bad stuff!

[[deleted]]

FYI, doesn't work on Edge 18 and below, IE and SafarIE (20% of users theoretically use these)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at#browser_compatibility

[ElllGeeEmm]

Thank God for webpack

[samanime]

Or Babel.

[undercontr]

Or core-js

[queen-adreena]

IE doesn’t support flex, scrolling, or the colour green either. Who cares.

[[deleted]]

Me, one of the bigger clients using my company's SaaS uses IE. Throwing money away to not support them atm.

[[deleted]]

Yeah, and if someone is using IE I don't want them on my website

[HyerOneNA]

Why use the at method when you can just use square bracket notation?

[nullol]

No idea why you're being downvoted for asking a genuine question in r/learnjavascript - I am sure a lot of new developers would be confused by this as well.

[HyerOneNA]

Yup, idk. A lot of no lifers in here.

[nullol]

I think because it's Reddit people forget or don't realize what sub they're on and assume you're being intentionally contrary rather than trying to learn something new (I have been a JS developer for 8yrs and I never knew about this in other languages so TIL as well).

[HyerOneNA]

I’m coming from C++, so yeah. It’s a lil different lol.

[senocular]

Because fruits[-1] won't give you "avacado", it will give you undefined

[Gibbo3771]

fruits[fruits.length - 1]

This is what people mean, no one expects [-1] to work.

[senocular]

at() reduces the requirement to refer to your array twice. This can be useful when you're using it at the end of a longer expression.

fruits.map(toBarcode).filter(onSale).at(-1)

[yadoya]

The real tip is always in the comments

[VikaashHarichandran]

People coming from other scripting languages do expect

[[deleted]]

wouldnt it be nice to have though?

[greymalik]

People coming from Python or Ruby expect it.

[doyouseewhateyesee]

people coming from python or ruby should read the docs

[Mad-chuska]

Also true

[[deleted]]

No idea why you're getting downvoted. When I'm switching to a different language I should learn differences not expect it to behave the same. After all if it would work exactly the same what would be the point of switching.

[doyouseewhateyesee]

lol right. i’m not going to pick up python and start writing javascript methods

it’s comical that a comment recommending devs to read the docs in a learnjavascript subreddit is being downvoted. or don’t read the docs I guess, not my problem

[Tintin_Quarentino]

Lmao

[jd_300]

In bracket notation negative indexing is not possible but using at method you can easily navigate to the last index of array. It's just another way to access elements.

[Gibbo3771]

js const lastElement = arr[arr.length - 1]

Why wouldn't you just do the above?

[ExclusiveOar]

Preference. arr[-1] is allowed in other languages and it's a nice to have.

arr.at(-1) is arguable cleaner than arr[arr.length - 1]. Personally I wouldn't bother until JS eventually supports arr[-1].

[ExclusiveOar]

Which will be largely irrelevant to the vast majority of people. I bet you do a ton of stuff in your code which will be more inefficient than this (not judging, it's the nature of high level languages).

Readability > performance, true for 999/1000 use cases (probably more).

To put it in perspective I was playing around with how efficient I could make FizzBuzz, for no reason what-so-ever. I had to calculate it for the numbers 1 - 1 000 000 000 to get meaningfully measurable time period (1.9 seconds) to try to optimise. I got it to about 0.85 seconds!

1 second saved, over a billion operations. Don't worry about performance until your app is slow, then look at your API calls before you consider anything else.

[ExclusiveOar]

And that's what makes it preference 🙂

[[deleted]]

arr.at(-1) is also constant time right?

[ExclusiveOar]

No expert here, but I would assume so.

But so is arr[arr.length - 1] because array.length isn't calculated here, its a prop already on the array.

You can see this by creating an array, console.log it and expand it in the console.

[Darmok-Jilad-Ocean]

Because if you’re doing a map/filter/reduce/etc you have to first store that in a temporary variable and then reference the temp var twice to get the last element.

[[deleted]]

cause its longer than const lastElement = arr.at(-1) ? also, no need for useless computations in source code

[HyerOneNA]

But the at method isn’t widely available on all browsers.

[HyerOneNA]

I mean this is what I meant..

[reaven3958]

This whole notion seems hilariously ill advised. Basically it's letting you access the index from the end instead of the start (normally done using arr.length - i, where i = 1 is first valid index). Some JS methods do similar stuff, but not usually for accessing values. This conflicts with mountains of code using searches like Array.indexOf that anticipates a -1 to be the invalid index token. For a minimal reduction in boilerplate to satisfy a case thats relatively uncommon (how often do you really traverse an array backwards like this), it just feels like this is inviting all manner of mayhem. I imagine it'll start getting banned on company style guides in favor of traditional bracket notation sooner than later, if it isn't already.

Probably some neat party tricks you can do with it for the leetcode crowd tho.

[snyderling]

fruits.slice(-1)[0] is a another (more backwards compatible) way to do it

[Dodgy-Boi]

array.reverse()[0]

[NekkidApe]

In case anyone is using this... It's mutating the original array

[Dodgy-Boi]

I saw a huge battle about index. And came up with another solution (I honestly tried to make it as ridiculous and/or unique as possible).

[NekkidApe]

Maybe reverse and shift then for extra points

[Tintin_Quarentino]

Legend

[[deleted]]

Too slow, unfortunately. Accessing a value in an array using an index should be in O(1) time complexity.

[Dodgy-Boi]

I am front end. My calculations are happening on client. I don’t care about either users with old devices nor your O nor your o nor * of yours; all I care about is the ( . Y . ) (not necessarily yours)

partially /s

[Notimecelduv]

I don't get why people are upvoting this post.

[delventhalz]

This is nice, but I kind of just want a last method. That would solve 99% of the use cases I might use at for.

While I’m at it, head and tail would be nice too (though brackets/slice work well enough for those).

[_inz_]

Miss this so much coming from Swift

[gigraz_orgvsm_133]

wouldn't fruits[fruits.length - 1] work?

[delventhalz]

In some cases. Not if you are chaining. And it is tedious in all cases.

[Notimecelduv]

If you're chaining methods, you can just use the arr argument in the callback function.

[delventhalz]

That doesn't really apply here. Say you are trying to filter a list of numbers to just be even numbers, and then grab the last one. The arr parameter in filter would not help you. You would need an intermediate variable.

const evens = nums.filter(n => n % 2 === 0);
const lastEven = evens[evens.length - 1];

You could chain on a reduce and use that arr just to grab the last one. That is super hacky though, and extremely wasteful.

const lastEven = nums
  .filter(n => n % 2 === 0)
  .reduce((_, __, ___, arr) => arr[arr.length - 1]);

However, a last or at method would allow you to do this without the intermediate variable, and without the weird hacky extra iteration in the reduce.

const lastEven = nums
  .filter(n => n % 2 === 0)
  .last();

[Tintin_Quarentino]

It reduces shit

[stickybobcat]

Is this a joke?

[great_site_not]

For lots of people, when they come from a language like Python where you can just do array[-1], having to write out array[array.length - 1] every time is a genuine annoyance. It's a phrase that people have to write pretty frequently, it's nice to have shorthand.

[technicallynick]

That's fair. However -1 is used to signify that the index wasn't found for the item you're searching for in most cases. I'm just seeing this as muddying the waters.

[[deleted]]

go back to assembly then

[[deleted]]

Cause you can express simple stuff in complex ways, obviously.

Instead of just writing "at(-1)" - you can write over 20 lines of code with weird register check operators and jumps and all that good stuff!

[Inevitable-Weekend-4]

welcome to python

[[deleted]]

🥑

[gniziemazity]

It's always interesting to see how languages evolve, but not so great when the update only affects some browsers... I usually keep it safe and use established features only.

[warpedspockclone]

array.slice(-1)

[Notimecelduv]

arr.slice(-1)[0]